1. Field of the Invention
This invention relates to a separation method of a semiconductor layer, and a production method of a solar cell using the semiconductor layer separated by the above separation method.
2. Related Background Art
The greenhouse effect gases, e.g., carbon dioxide and nitrogen oxides resulting from burning oil at thermal power plants and gasoline by vehicles, are polluting the global environments. Therefore, power generation by solar cells has been increasingly attracting attention, viewed from the above environmental concerns and anticipated depletion of crude oil.
The solar cell of thin-film crystal silicon (Si) has various advantages; it can be produced at low cost, because of its thin power generation layer which decreases required quantity of a Si raw material, the power generating layer of crystal Si can exhibit a higher conversion efficiency and durability than the other types of solar cells, e.g., that using amorphous Si, and it can be bent to some extent, which makes it applicable to curved surfaces, e.g., car bodies, home electric appliances and roofs.
Japanese Patent Application Laid-Open No. 8-213645 discloses a method of separating thin-film single-crystal Si using an epitaxial layer grown on a porous Si layer, in order to realize a solar cell of thin-film crystal Si. FIG. 23 shows a schematic section explaining the method of producing the solar cell of thin-film Si, disclosed by Japanese Patent Application Laid-open No. 8-213645, where 101: Si wafer, 102: porous Si layer, 103: p+ type single-crystal Si layer, 104: pxe2x88x92 type single-crystal Si layer, 105: n+ type single-crystal Si layer, 106: protective layer, 109 and 111: adhesives, and 110 and 112: jigs. In the method of producing solar cell, shown in FIG. 23, the porous Si layer 102 is formed on the surface of the Si wafer 101 by anodization. Then, the p+ type single-crystal Si layer 103 is epitaxially grown on the porous Si layer 102, and then the pxe2x88x92 type single-crystal Si layer 104 and n+ type single-crystal Si layer 105 are grown thereon, in this order. The protective layer 106 is further formed thereon. The protective layer 106 and Si wafer 101 are adhered to the jigs 112 and 110, respectively, by adhesives 111 and 109. A tensile force P is applied to the jigs 112 and 109, to separate the Si wafer 101 from the single-crystal Si layers 103, 104 and 105 as epitaxial layers at the porous Si layer 102. These single-crystal Si layers 103, 104 and 105 are used to produce a solar cell. The used Si wafer 101 can be recycled to reduce the production cost.
One of the methods of forming single-crystal or a polycrystalline Si is a liquid-phase growth method. This method can produce at a low cost a thick Si layer needed as the power generation layer for solar cells, in comparison with the other methods including a chemical vapor deposition (CVD) method. U.S. Pat. No. 4,778,478 discloses a concrete example of the method of liquid-phase growth. FIG. 21 shows a schematic section of the slide type apparatus for liquid-phase growth, disclosed by U.S. Pat. No. 4,778,478, where 50: slide board of a refractory material, e.g., graphite, 54 and 56: solvent reservoirs, 58: movable slide of metallic substrate, 60: cavity at the boat bottom, 63: barrier layer, 68 and 70: solvents, 72: portion to which a transparent electroconductive electrode is attached, 75: nozzle for forming an antireflection layer, 74: chamber for containing the nozzle 75, 76: wheel, and 78: nozzle for forming the barrier layer 63. This apparatus first unwinds the movable slide 58 wound like a roll on the wheel 76, and forms the barrier layer 63 by the nozzle 78. It then forms the semiconductor layer as the power generating layer, grown in liquid phase from the solvents 68 and 70 in the reservoirs 54 and 56; the transparent electrode at the portion to which the transparent electrode is attached; and the antireflection layer by the nozzle 75, to produce the solar cell. This method efficiently effects the slide type liquid phase growth, and is advantageous for mass production of solar cells.
U.S. Pat. No. 5,544,616 discloses a dipping type apparatus for liquid-phase growth. FIG. 22 shows the schematic section of this apparatus, where 201: outlet, 202: quartz crucible, 203: boat of graphite, 204: heater, 205: nozzle from which argon gas is injected, 206: thermocouples, 208: lid, 209: insulated region, and 210: supporting table of graphite. This apparatus grows a semiconductor layer on a substrate by dipping the substrate in the solvent held in the quartz crucible 202.
Japanese Patent Application Laid-Open No. 8-46018 discloses a method, in which a wafer is supported by adsorbing its back side on a table via an ice layer. In this method, since the wafer was diced with a low-elasticity dicing tape attached to its back side, strain was generated in the wafer during the dicing step, thereby leading to chipping and cracking of the wafer. Therefore, this application discloses that use of a high-elasticity ice layer for the support of the wafer prevents the above problem and dispenses with a dicing tape, thereby improving dicing efficiency.
Japanese Patent Application Laid-Open No. 8-213645 discloses a method of separating a single-crystal Si layer from an Si wafer, in which the single-crystal Si layer adhered to a jig 112 by an adhesive 111 via a protective layer 106 is separated from the Si wafer 101 whose back side is adhered to a jig 110 by an adhesive 109 by pulling the jigs 112 and 110 in the opposite directions to mechanically destroy a porous Si layer as the separation layer. This publication also discloses that the jig 110 is removed from the Si wafer 101 to which the jig is adhered by an adhesive 109, and the Si wafer 101 is reclaimed for recycle. However, it is necessary for the adhesive 109 to fast adhere the Si wafer 101 to the jig 110, in order to transfer a high tensile force to the porous Si layer 102 in a separation step, which makes it difficult to later remove the jig 110 from the Si wafer 101. In other words, it is difficult to completely remove the adhesive 109 from the Si wafer 101, and the Si wafer may be damaged during the removal step.
It is an object of the present invention to provide a separation method of semiconductor layer which fast supports the layers during the separation step, allows them to be easily separated from the jigs after the separation step, and brings about advantages, e.g., prevention of damages of the substrate, to facilitate the subsequent steps. It is another object of the present invention to provide a production method of a solar cell using the above separation method.
The inventors of the present invention have intensively studied in order to solve the above problems and accomplished the following inventions. The first separation method of a semiconductor layer according to the present invention is a method for separating a semiconductor layer and a semiconductor substrate which supports the semiconductor layer from each other at a separation layer formed between them, wherein a face of the substrate at a side opposite to a separation layer is held by utilizing an ice layer. The second separation method of a semiconductor layer according to the present invention is a method for separating a semiconductor layer and a semiconductor substrate which supports the semiconductor layer from each other at a separation layer formed between them, wherein a face of the semiconductor layer at a side opposite to a separation layer is held by utilizing an ice layer. These two inventions may be effected simultaneously. The production method of a solar cell according to the present invention comprises using the semiconductor layer separated by the above-mentioned separation method as a photoactive layer for solar cells.
The separation method of the present invention is characterized in that the ice layer is used not only for holding the semiconductor layer or the semiconductor substrate, but also for cooling the separation layer and positively imparting stresses due to expansion or contraction to the substrate, the separation layer and the semiconductor layer, thereby uniformly breaking them, or cooling the separation layer to decrease its strength. This allows the semiconductor layer and the substrate to be more easily and uniformly separated from each other.
The above effects is more remarkably exhibited, when a porous layer is used as the separation layer. A porous layer used as the separation layer is required (1) to be easily separated, i.e., to have a fragile structure, and (2) to stably have a strength prior to the separation step, which tend to run counter to each other when the semiconductor layer is not surely supported by the porous layer prior to the separation step, problems may occur. For example, they may be separated from each other while a device is assembled, to possibly produce defective products. On the other hand, when the semiconductor layer is supported by the porous layer with a excessive force, the semiconductor layer may not be easily separated during the separation step, or the separated semiconductor layer may be cracked. It is therefore necessary for the porous layer to have an adequate strength, but controlling its strength is not easy.
The separation method of the present invention utilizing ice to hold the semiconductor layer or the substrate during the separation step cools the porous layer to decrease its strength, thereby simultaneously achieving stability of the porous layer prior to the separation step and easiness of its separation during the separation step.
The porous layer is held preferably at xe2x88x925xc2x0 C. or lower, and more preferably xe2x88x9221xc2x0 C. or lower in order to easily carry out separation. At the same time, the holding temperature is preferably xe2x88x92220xc2x0 C. or higher, in order to prevent loss of strength of the semiconductor substrate and the semiconductor layer.
In this invention, it is preferable that the substrate which supports the semiconductor layer is a Si wafer; the separation layer is a porous Si layer prepared by anodizing a Si wafer, the semiconductor layer is a single-crystal Si layer epitaxially grown on the porous Si layer; and a holding means is closely adhered to the ice layer. For the holding means, a flexible, film-like substrate or hard substrate is suitably used. It may be an ice plate made of ice itself, or a porous spacer which adsorbs water. The holding means may be provided with a cooling mechanism inside. The semiconductor layer may be adhered to a supporting substrate of different thermal expansion coefficient, the semiconductor substrate may be held while cooling the semiconductor substrate, and the difference in thermal expansion coefficient may be utilized to carry out separation.
It is preferable to produce a solar cell using the semiconductor layer, separated by the method of the present invention, as the active layer for the cell. In addition, the semiconductor layer may be used as the active layer for general semiconductor devices, e.g., sensors and liquid-crystal displays.